Squelch circuits are used to preclude lower level or noisy signals from entering a communications processing circuit or system. A familiar example is a hand-held transceiver radio, e.g. a walkie-talkie or CB radio. Without squelch circuits, these systems would generate a rather annoying hissing sound when no signal is received. With squelch control, input signals are passed to the speaker only when the input signal exceeds a prescribed level. In some implementations, the squelch level is adjustable, for example, by a person manipulating a potentiometer configured to provide a reference potential for squelching.
In the related, commonly assigned, U.S. patent application Ser. No. 08/974,672, entitled "Signal Detection Circuit and Methodology" filed on Nov. 19, 1997, I describe an improved multi-level quantizer using delay line techniques. Therein, delay lines capture and buffer timing information for those portions of an analog input signal that are at or above certain amplitude levels, determined by comparing the input signal with a corresponding reference potential from a voltage divider network. Therefore, certain amplitude levels of the input signal are precluded, i.e. squelched, from entering a delay line, thus directing various amplitude signals into appropriate delay lines.
Referring to FIG. 1, a multi-level quantizer 100, described in the related application, provides a snapshot of both amplitude and timing (e.g. phase or frequency) information of an input signal, received at node V.sub.in. Appropriate pattern detection logic 140 may be used to demodulate or detect the input signal based on the snapshot provided by the multi-level quantizer 100.
In this circuit, an input signal is received at node V.sub.in and applied to a plurality of comparators 122, 124, 126, and 128 for comparison with a respective, different reference potential received from a voltage divider 110. Voltage divider 110 is a chain of resistive elements 112, 114, 116, 118, and 120, e.g. resistors, coupled in series from a source of a reference supply potential V.sub.ref, e.g. a system supply potential V.sub.cc, to a source of ground potential. If the voltage drops across resistive elements 112-120 are equal in the illustrated configuration, then the highest reference potential is four-fifths of the reference supply potential and the lowest reference potential is one-fifth of the reference supply potential. For example, with a reference supply potential of 5 V, the reference potential supplied to comparator 128 is 1 V.
The output of each comparator 122-128 is coupled to the input of a respective delay line 132-138 for repeatedly delaying an output pulse of each comparator for a common delay period. In this manner, the comparators 122-128 quantize the amplitude information of an input signal according to a reference potential by generating a pulse based on a comparison of the input signal and the respective reference potential. The delay lines 132-138 buffer and hence capture the timing information of the pulses from the comparators 122-128. Thus, pattern matching logic 140 coupled to the delay lines 132-138 is able to synoptically inspect an analog signal that is quantized into pulses and buffered for signal detection or demodulation.
In some operating environments, however, it may be disadvantageous to use a voltage divider for providing the plurality of reference voltages. For example, when the squelch level is lowered, the lowest reference voltage, a fraction of the reference supply potential V.sub.ref, may be brought down to a level that causes difficulties in the multi-level quantizer 100. For example, a comparator may have a threshold voltage of 0.7 V, below which signals cannot be compared. Consequently, amplitudes in the input below the threshold voltage are not detected by the quantizer. Thus, the normally negligible theshold voltage of a comparator becomes a constraint limiting the range of squelching levels. Moreover, comparators using a low reference potential generated by the voltage divider 110 may be more susceptible to noise.
In low power situations, a voltage divider may consume more power than desirable, because it provides an electrical conduction path that is always conducting between sources of supply and ground potential. In another example, it may be difficult to manufacture resistors or other large-lumped resistive elements of the appropriate precision in some implementations, e.g. on monolithic semiconductor substrates with highly miniaturized components.